Our long-term goal is a better understanding of how important metal containing enzymes work. In this proposal, we focus on metalloenzymes that contain both Fe and a different metal (Ni, Mo, V) at the active site. Three of the enzyme active sites (NiFe hydrogenase, CO dehydrogenase, and acetyl-CoA synthase) contain combinations of Fe and Ni. The other enzyme, nitrogenase, employs V or Mo at one end of a MFe7S8 cluster. Despite progress by x-ray diffraction (or electron microscopy) techniques, many questions about the molecular and electronic structure need to be resolved. The catalytic mechanisms are still poorly understood. The molecular structure of the Ni sites will be studied using extended x-ray absorption fine structure (EXAFS) analysis of crystals of H2ase and CODH. Samples containing only the CO oxidation (C-cluster) and ACS (A-cluster) sites will also be examined. Although these enzymes have already been studied by EXAFS, the proposed samples and range-extended analysis techniques should yield more details about the Ni sites. The electronic structure of the Ni and V sites will be probed using x-ray resonance Raman spectroscopy (XRRS or RIXS). Analysis of these spectra will reveal the best description of the Ni and V oxidation and spin states under a variety of conditions. The Fe- and Ni-centered vibrational density of states will be studied using the vibrational Mossbauer spectroscopy. This approach will help identify the sites of important M-H and metal-substrate interactions. An instrument will be built to allow on-campus experiments. The above enzymes are important for maintaining our environment. Fe proteins also play key roles in human health and disease. The proposed work will enhance our knowledge of Fe biochemistry, and the spectroscopic techniques under development will have applications to a wide range of bioinorganic problems.